Clinical testing and characterization of one lot of plasma-derived hepatitis B vaccine prepared by the NIAID have been completed. The vaccine was highly immunogenic, safe and well tolerated when tested in healthy persons ranging from infants to adults. Results from an efficacy trial in Asia indicate that the vaccine effectively prevents transmission of hepatitis B virus infection from mothers to infants. The sera from these infants are being analyzed for responses to specific HBsAg epitopes in an attempt to identify those that are most important.